1. Field of the Invention
The invention relates to torque transmitting couplings for drive lines, and in particular, to a torque overload clutch for a drive line.
2. Description of the Related Art
A driven shaft of an agricultural implement is connected to a power take-off shaft of a tractor by at least one universal joint to ensure the proper translation of torsional forces between the drive shaft and driven shaft. When attaching an agricultural implement to a tractor, it is often desirable to limit the torque which can be transmitted by the drive line. Failure to do so may result in excessive torsional forces, resulting in damage to the shafts or other elements of the implement or tractor.
Different types of torque limiting clutches are known, for example, friction, shear and detent clutches. Friction clutches transmit torque through a frictional interface and tend to wear at an undesirable rate and produce heat when the clutch disengages. Additionally, typical friction clutches don""t warn or signal the operator when they disengage. As a result, the clutch may be worn out or time may be lost during operation when the user is under the false impression that the agricultural implement is operating as expected.
Previous methods to limit torque additionally include shear devices in which a shear pin is sheared off when the torque exceeds a certain level. In such devices, a pin or bolt is sheared and must be replaced each time the torque limit is exceeded.
Detent clutches provide position engagement, alert a user when the torque threshold is exceeded, and require little or no user intervention to render the device once again operational after the torque threshold has been exceeded. U.S. Pat. 5,733,196 discloses one such device which discloses a coupling having an outer housing attached to one of the shafts, and an inner housing attached to the other. Specifically, the inner and outer housings become engaged when a set of pawls move from an inner position, in which they are entirely within an opening in the inner housing, to an outward position, in which the pawls are partially disposed in the opening as well as in a recess in the outer housing, such that the two shafts are engaged to rotate together. When the torsional forces between the two shafts exceed a predetermined threshold, the pawls cam radially inward to a disengaged position against the force of a disc spring which acts axially. Once the clutch is disengaged, a radial spring moves a multi-part segmented switching ring to block re-engagement of the pawls with the recess so that the pawls do not re-engage until the speed is at a sufficiently low level, to avoid overly abrupt or jarring re-engagement.
The present invention provides a torque limiting clutch that avoids overly abrupt or jarring re-engagement without introducing additional parts to the mechanism. In a clutch of the invention, the spring member is a regressive disc spring whose resistance decreases dramatically when compressed past a certain point, as occurs when the clutch becomes disengaged. The small residual spring force when the clutch is disengaged is sufficient, however, to bias the driving members radially outwardly to again engage the clutch with a relatively small initial re-engagement force when the relative speed and torque between the drive and driven shafts is sufficiently small or zero. The re-engagement force is also sufficient to cause the driving members to chatter as they pass by the recesses in the outer member, so as to warn the operator that the clutch has become disengaged.
In a preferred form, the spring is preloaded in the engaged position to a force which is short of the maximum force, for example 40-80% of the maximum force the spring is capable of, which typically occurs at a deflection of less than 50% of the height (height=total lengthxe2x88x92thickness) of the spring. The maximum force typically occurs at about 50% deflection, so initially the force exerted by the spring goes up as the driving members move radially inward toward disengagement. After the maximum force is passed, the force goes down, and can go down significantly if the deflection beyond the maximum force is sufficient. Hence, the disc spring preferably can deflect to and beyond the flat shape, so that it concave in the disengaged position, exerting only a very small force to bias the driving members back into re-engagement.
Preferably, the height to thickness ratio of the spring is chosen to be as high as possible, while still permitting the spring to return after being deflected past flat to a concave shape. Thus, the height to thickness ratio should be in the range of about 2.3 to 2.8, and preferably is between 2.6 and 2.8.
In another aspect of the invention, an actuator ring cams on the driving members and is biased by the spring to urge the driving members into the recesses of the outer housing. On the side of the spring opposite from the actuator ring, a compression plate retains the spring axially. The compression plate has a concave or other radially inwardly receding surface which permits the spring to assume a concave shape in the disengaged position. The compression plate also prevents the spring from being damaged by being overly flexed into a severely concave shape.
These and other features and advantages of the invention will be apparent from the detailed description and drawings.